Betray one's principles in very popular show (4-3)
I believe the answer is:
sell-out
I believe this is a double definition.
'betray one's principles' is the first definition.
(I've seen this before)
'very popular show' is the second definition.
(a sell-out show is very popular)
'in' is the link.
(Other definitions for sell-out that I've seen before include "successful show" , "Betrayal" , "When all tickets are booked in advance" , "Event with no further ticket availability" .)